Alkenes and dialkenes (ethene, propene, butene, iso-butene, iso-prene, butadiene, etc.) are extensively applied to synthetic resin, plastics, high-octane-rating gasoline blending ingredients (methyl tert-butyl ether, methyl tert-amyl ether and alkylated oil) and other high additional value products. These alkenes are produced through the processes, such as hydrocarbon steam cracking (such as ethane steam cracking and naphtha steam cracking), alkene catalytic cracking (such as Superflex), heavy oil catalytic cracking (such as TMP and DCC) and heavy oil catalytic pyrolysis (such as CPP). And It is also an important technical route to prepare alkene and dialkene via the alkane catalyzed dehydrogenation.
As an important way for producing high-added-value low-carbon alkenes by reasonably using rich and low-carbon alkane resources, alkane dehydrogenation is increasingly taken into account by people.
Dehydrogenation of alkane is a relatively-strong endothermic reaction, for example, dehydrogenation of propane and iso-butane,C3H8→C3H6+H2 ΔH°=124.3 kJ/moli-C4H10→i-C4H8+H2 ΔH°=117.6 kJ/mol
Reaction heat of the dehydrogenation of the propane and reaction heat of the dehydrogenation of the iso-butane separately reach 124.3 kJ/mol and 117.6 kJ/mol at a temperature of 25° C. under a pressure of 0.1 MPa. The problems, such as what kind of reactor to be adopted and how to effectively supply heat to reactions, must be thought seriously.
Dehydrogenation reactions of alkanes are limited by thermodynamic equilibrium. Under the same temperature conditions, the larger molecules of alkanes are, the higher the equilibrium conversion ratio is; and for the same kind of alkanes, the higher the temperature is, the higher the equilibrium conversion ratio is. For ethane dehydrogenation to ethene, if a catalytic dehydrogenation method is adopted, the dehydrogenation is limited by the thermodynamic equilibrium, the conversion ratio per pass is too low. Thus, the dehydrogenation of the ethane adopts a steam pyrolysis technique at present, and the reaction is performed under high-temperature conditions with the temperature of 800° C. or above. For catalytic dehydrogenation of propane, butane, etc., the conversion ratio per pass and alkene selectivity are economically feasible under proper temperature conditions, so that a catalytic dehydrogenation method is generally adopted to prepare propene from propane, and butene or butadiene from butane.
In the aspect of catalytic dehydrogenation reactor, fixed bed, moving bed and circulating fluidized bed are all applied. Alkane dehydrogenation catalysts are liable to deactivation by coke formation, and Pt is liable to sintering in case of a Pt-based catalyst, thus the catalysts need frequent coke-burning regeneration or oxychlorination regeneration. As for a fixed bed, regeneration is inconvenient apparently; and in case of a moving bed and a fluidized bed, reaction and regeneration can be carried out continuously. The Pt-based catalyst is expensive, the fluidized bed can only employ Cr-based catalysts, and the Cr-based catalysts can cause serious pollution to environment. The moving bed adopts the Pt-based catalyst, and reaction is required to be carried out in the presence of hydrogen in order to guarantee that the catalyst has a regeneration period of several days. As a result, the conversion per pass will be lowered. And due to the lower conversion per pass and hydrogen circulation, the energy consumption of the moving beds can be very high.
In the aspect of catalyst regeneration, heat transfer efficiency and reaction efficiency, an optimal reactor for alkane dehydrogenation is a circulating fluidized bed obviously. Process flows performed in a circulating fluidized bed reactor are much simpler than those of the fixed beds and the moving beds, and the investment is lower in case of devices of the same scale. A focus of contradiction lies in the development of non-toxic and relatively-cheap catalysts capable of being applied to the fluidized bed and the mating of a circulating fluidized bed reactor according to properties and performance of the catalysts.
For a circulating fluidized bed reaction device for alkane dehydrogenation, a pursued objective in the field is to increase the conversion per pass and alkene selectivity forever. Whereas, in the reaction device, back-mixing phenomenon of gas phase is also one of factors affecting the selectivity and conversion for alkane dehydrogenation to alkene.